It has been a long standing manufacturing problem to efficiently produce ceramic monoliths. Ceramics with skin related flaws, known in the trade as "fissures," account for approximately 37% of all the rejects. The fissures are focal points of stress and heat differentials in the ceramic support during automotive exhaust gas cycling. These small cracks cannot be tolerated because they eventually spread and cause failure of the catalytic support material.
From the beginning of ceramic substrate manufacture, fissure formation has been a constant source of perplexity. The fissure problem became even more exacerbated when the extrusion system was upgraded in the plant from a ram fed, batch-type process to a screw fed, continuous-type process. The continuous-type process created a higher percentage of fissure related rejects. The increase in failures forced more attention to be given to the elimination of the problem, with the result that the cause of the difficulty was finally ascertained.
The ceramic material leaves the extruder die as a long tubular mass, referred to as a "log." As the log leaves the extruder, it is conveyed by air bearings to a ceramic carrier contoured to the shape of the log. The ceramic logs are dielectrically dried, wherein the water molecules in the material are converted to steam that readily escapes through the porous ceramic structure. The dried logs are then cut and fired to form the catalyst material supports.
It is during the travel to the ceramic dryer carrier that the surface of the log cracks. The log leaving the extruder is usually warmer than the ambient air. Water evaporation from the substrate begins immediately upon its exit from the extruder die. The underside of the log dries faster than the rest of the material by virtue of the air jet convective drying of the undersurface of the ware. It is theorized that a stress differential is created between the top and bottom surfaces of the log causing fissures to form, especially on the top surface. The fissures form to alleviate the stress created between the quicker shrinking bottom surface layer and the top layer.
It was suggested that the humidity be raised to 100% in the air being forced through the air bearing jets. This, however, did not prove a satisfactory solution. The ceramic material having a high percentage of fine clays becomes tacky when its surface is wetted. The tacky surface is then difficult to convey over the air bearings to the dryer carrier.
Rather, it was discovered, only a given range of air jet humidity provides satisfactory results. Too wet, and the log became tacky and hard to convey over the air bearing surfaces. Too dry, and fissures continue to form on the log.
Also, it was observed, differential shrinkage would still persist where the log was not jacketed to prevent the moist air from escaping from about the surfaces of the log. Thus, a mantle was built about the air bearing support to confine the humidity to the vicinity of its application.
The relative humidity of the air flow in the bearings necessary to prevent fissuring has been observed to lie within well defined limits. The relative humidity has to be within the approximate range of between 85 to 97%, with a preferred operating range approximately being between 90 and 95%. Within this relative humidity range, the temperature of the air, however, has not been found to make a significant difference in the fissure phenomenon. The general temperature range for the air during the humidity testing fell between 65.degree. and 105.degree. F.